This invention relates to an improved bearing system for a rotary stage.
Typically, rotational stages consist of a rotor pivoted by two bearings mounted along the axis of a stationary shaft. Wobble is introduced to the rotating element due to the normal out-of-roundness of the balls in even the best of ball bearings. Typical out-of-round/out-of-size ball-to-ball in a high grade bearing is about 10 xcexc-in. This results in a true-center wobble of 4 arc-seconds using a worm gear of 1 inch diameter. This transaxial wobble is usually acceptable. But, the axial wobble is a result of the compounding of the errors of both bearings, taken over the distance between the bearings. A stage which has a low profile will suffer greater axial error/wobble from the shortness of that distance.
An object of the present invention is to overcome the problems associated with the prior art and to provide a device which improves the angular-wobble of the rotor four to eight fold, i.e. decreases the error from 20 arc sec. down to 1 arc sec. or better, while converting random wobble to smooth consistent deviation within the error noted.
According to the invention, there is provided a rotary stage comprising a rotary table mounted by an adjustable axial preloaded bearing on a shaft defining a table axis of rotation and supported on a base plate by an optically polished surface.
The polished surface may be formed on the rotor, on the base plate or on both of these. When a single said polished surface is utilized, the corresponding contacting surface may comprise three low friction (e.g. Teflon) pads symmetrically disposed about said axis. The mating polished surfaces, when used, may have air under pressure supplied between them to form an air bearing.
The bearing on the shaft may, for example, be a ball, air, roller, single point bearing.
The table may have an optically flat polished upper surface and may incorporate a worm gear for engagement with a worm to control rotation of the table about the axis.
Preferably the invention provides a rotary stage having a rotary table mounted by a bearing on a shaft defining a table axis of rotation and supported on a base plate by an optically polished surface, the bearing being pre-loaded axially by an axially adjustable split ring in engagement with an annular chamfer on a race of the bearing to center the bearing relative to the table axis.